Why So Blue?

First things first—these blue testicles are not due to sexual frustration.

The colour is also not caused by hormonal shifts, as in the case of the red genitalia seen in baboons and other primates, Fred Bercovitch, a wildlife biologist at Kyoto University in Japan, says via email.

Though the blue pigments are not completely understood, they're likely linked to sexual selection, Bercovitch says, though in mandrills, colour has been linked to social status.

Male mandrills, native to rain forests of equtorial Africa, have vivid red and blue facial colours that match the eye-catching colours on their hindquarters. The brighter the face, rump, and genitalia, the higher the male’s rank, which a 2005 study showed could sometimes help avoid costly conflict. What's more, female mandrills prefer males with more vibrant colours.

Male vervets of East Africa that have more intense blue scrota are "more likely to be aggressive with and bully juvenile males," says Jennifer Danzy Cramer, a biological anthropologist at American Public University in Charles Town, West Virginia.

Vervets, which live throughout sub-Saharan Africa, are the most wide-ranging of the African monkeys [Image: Goncalo Diniz, Alamy Stock Photo]

Vervets also like to show off their bonnie blues, adds Kyoto University's Bercovitch, unlike mandrills and patas, a primate native to the central African grasslands.

Overall, Bercovitch says greater contrast and larger size "are probably alluring traits" (think the eye-catching peacock's tail) so those males with the most vibrant and biggest scrota attract females. For instance, patas testicles can grow to twice their size during mating season.

Family Jewels

So how do the primates actually get their blue junk? At a molecular level, the colour originates from the Tyndall effect, the scattering of light by the skin itself, Bercovitch says. The skin of blue-hued monkeys also has unusually neat and orderly collagen fibres, according to a 2004 study. They're so well organized that a change of as little as a millionth of an inch in size of or distances between would produce a different colour.

Speaking of, the monkeys can take on different hues.

The lesula, an African primate, only formally discovered in 2007, has a bright blue scrotum and buttocks that turns white when the animal dies.

Patas have aquamarine scrota, while many adult vervets' are more turquoise.